Yahoo disclosed that 1 billion user accounts was hacked in August 2013. This breach is different from the 500 million user accounts that was stolen from its network in late 2014.
Yahoo user names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5), encrypted or unencrypted security questions and answers were stolen in 2013 breach according to Yahoo.
Since the announcement, Yahoo started showing pop-up alert to users whose account might have compromised.. here is the screenshot of the alert.
Yahoo Security Notice December 14, 2016 ( here )
Yahoo press release about August 2013 breach